In the age of continuous depletion of the oil resources, recently studies on synthesizing a polymer precursor from biomaterial as opposed to oil resources lively continued. In particular, cadaverine (1,5-diaminopentane) having two amine groups at the two terminals and consisting of five carbon atoms, is a material for use as a polymer precursor, and moreover, for various application in the field of chemical industry. The examples include polyamides, polyurethanes, and chelating agents. As a part of the studies, a recent research on synthesizing cadaverine from lysine, or a research on producing cadaverine in mass from glucose or starch which are even cheaper than lysine, using biotransformation function of microorganism cells is actively in progress. As such, there may be various methods for producing cadaverine in mass using biotransformation technology via gene mutation of a host cell, but the starting material being starch or glucose, lysine synthesis can be performed via several steps of biotransformation procedures, and this lysine can be transformed to other compounds via various reactions. Because of this, lysine is used as a main starting material. One example of this is decarboxylation, and cadaverine can be synthesized by lysine decarboxylase reaction. The technology of producing lysine from glucose in microorganisms can already produce about 120 g/L from Corynebacterium glutamicum, and the mass production is commercialized in practice. Accordingly, a technology of transforming the highly concentrated lysine as a substrate to various compounds in a cell, or a technology of converting to cadaverine more efficiently in vitro by one-step reaction is necessary.
In the case of lysine decarboxylase, it is not present within Corynebacterium glutamicum, but it is known to be present in variously derived microorganisms. Among them in particular, lysine decarboxylase derived from Escherichia coli using PLP (pyridoxal-5′-phosphate) coenzyme is known to have high reactivity. In Escherichia coli, two types of lysine decarboxylase are present, LdcC lysine decarboxylase which is continuously expressed when the cell grows, and LdcI lysine decarboxylase which shows induced expressions only in condition of weak acidic pH are present. Houry research team of Toronto University of Canada, revealed the structure of LdcI lysine decarboxylase derived from Escherichia coli, and also revealed that the enzyme is in a form of a decamer consisting of 10 units. By the revealed structure of lysine decarboxylase and the mutation of amino acid residues, it became possible to predict what is the important element affecting the enzyme activity, and how the reaction mechanism works.
There are several disadvantages for using lysine decarboxylase LdcI derived from Escherichia coli, which is relatively high in activity, in a highly concentrated industrial reaction. Whilst it is high in activity in weak acidity (pH5.6), it has disadvantage of rapidly losing activity in neutrality or alkalinity. Because of this disadvantage, in order to perform repetitive reaction over long time with high concentration, increase in pH, thermostability and higher enzyme activity are required for LdcI enzyme. For these reasons, it is necessary to improve the enzyme reactivity throughout the overall pH through structural studies of the enzyme, and develop a mutant strain with increased stability for heat and pH, and thereby apply them to highly concentrated and continuous lysine biotransformation.